West Howland wrote an answer about Sea To Summit eVent Compression Dry Sack on July 26, 2011
I would have to disagree with the two previous answers. Having spent a lot of my time setting up and taking down camp in the rain, keeping at least the body of your tent dry can be a huge part of keeping the rest of your setup (pad, sleeping bag, clothing, etc) dry. Setting up the fly, then floor, then body (in that order) makes this do-able. If the inside of your tent gets wet, everything that touches it will follow suit. If more of your stuff stays dry, the happier, warmer, and lighter you and your gear will be especially with a down sleeping bag.